Nanowires welded into a mesh, using light

Some day, meshes made from nanowires could be used in devices such as video displays, LEDs, thin-film solar cells, and touch-screens. According to research performed so far, such meshes would be very electrically conductive, cost-effective, and easy to process. What has proven challenging, however, is finding a way of getting the criss-crossed nanowires to fuse together to form that mesh - if pressed or heated, the wires can be damaged. Now, engineers from Stanford University may have found the answer ... just apply light.

The process utilizes plasmons, particles of oscillating plasma that are created when light strikes a metal surface - under the right conditions. When the Stanford team laid nanowires out on a metal platform, then subsequently illuminated it, plasmon activity was concentrated where the wires crossed one another.

"When two nanowires lay crisscrossed, we know that light will generate plasmon waves at the place where the two nanowires meet, creating a hot spot," explained Mark Brongersma, an associate professor of materials science engineering. "The beauty is that the hot spots exist only when the nanowires touch, not after they have fused. The welding stops itself. It's self-limiting."

Each top nanowire acted like an antenna, guiding the plasmon waves into the wire beneath it. This created heat, which caused the two nanowires to fuse together where they touched. The process didn't affect the rest of the wires, however, nor the underlying material.

Not only could this method be used to easily produce nanowire meshes, but it could also come in handy for creating mesh electrodes for use on flexible or transparent surfaces. To demonstrate this, the scientists sprayed a solution containing silver nanowires in suspension onto a piece of Saran Wrap, then allowed it to dry. After illuminating the surface, they were left with an ultrathin layer of welded nanowires. They then balled the plastic up like a piece of paper. When they smoothed it back out, the mesh maintained its electrical properties, and the coated plastic was still virtually transparent. The researchers believe that the technology could be used to apply inexpensive solar power-generating coatings to windows, that would still allow light to come through while reducing glare for people inside buildings.

According to Brongersma's colleague Dr. Erik C. Garnett, previous nanowire-welding techniques would have melted the Saran Wrap before affecting the silver nanowires.